#include <stdio.h>

int main(int argc,char *argv[])
{
	int a = 0,b,c;//a为总量，b为输入值，c为叠加量
	scanf("%d",&b);
	for(c=1;c<=b;c++)
	{
		a = a + c;
	}
	printf("sum = %d\n",a);
	return 0;
}